Award: OCE-2022984

Since the start of the Industrial Revolution over 250 years ago, humans have burned massive amounts of fossil fuels to provide energy. This has raised the concentration of carbon dioxide (CO2) in the atmosphere by over 50% and appears to be affecting the Earth's temperature and climate. Figuring out where all this excess CO2 is going is important in order to predict what the future holds. Photosynthesis in the oceans converts carbon dioxide into organic matter that settles into the deeper ocean at a rate that is comparable to the annual emission of carbon dioxide from fossil fuel burning and is therefore an extremely important part of the global carbon cycle. This is referred to as the Biological Carbon Pump (BCP). Like in a garden, the rate of marine photosynthesis is affected by environmental factors such as temperature and sunlight and the availability of the essential nutrients phytoplankton need to grow. The essential nutrients include what are called "macro-nutrients" like nitrate, phosphate, and silicate along with trace element  "micro-nutrients" such as manganese, iron, cobalt, nickel, copper and zinc which are required for most of the enzymes to function properly. Iron is especially important because so many enzymes, including those involved in photosynthesis, require iron. When the organic matter produced by phytoplankton sinks into the deeper ocean it carries some of those essential trace elements along with it, lowering their concentrations in the sunlit surface ocean. To keep the BCP running efficiently there must be some balance between the loss of these essential trace elements and inputs from sources near the surface. This main goal of this project was to figure out how much of those essential trace elements enter the upper ocean from the atmosphere, where dust (aerosols) from the land, rich in trace elements, is swept out over the open ocean and deposits when it rains. To measure the deposition rate of dust we used a naturally-occurring radioactive element, berylium-7, which is produced in the atmosphere by cosmic rays and deposits with the dust when it rains. Dividing the inventory of Be-7 in the upper ocean by its concentration in dust yields an estimate of the dust flux rate. We can then use the dust flux rate to calculate the flux of those essential trace elements. We collected daily dust samples and analyzed them for their trace element content but we recognize that not all of the dust will actually dissolve in the ocean so we do experiments to measure what fraction of the essential trace elements might become soluble in the upper ocean. Thus, we can calculate the flux of soluble aerosol iron (or any other trace element) and compare it to the removal rate due to the sinking of organic matter produced from phytoplankton production. If the input and removal rates are similar then we conclude that the ecosystem is at "steady-state" and functioning correctly. This is what we saw in the tropical and subtropical waters of the Southwestern Indain Ocean. If the removal rate is greater than the input rate then it is possible that the ecosystem will "run down" over time as the concentrations of those essential trace elements get lower and lower. This is what seems to be happening in the subpolar and polar waters closer to Antarctica and suggests that we needed to search for other sources of those essential trace elements. One likely additional input source is from the volcanic islands that are common in this region and, in fact, the currents sweeping around those islands provide significant amounts of soluble iron to "fuel" phytoplankton production. Measuring the balances (or imbalances!) between these trace element input and removal rates provides critical knowledge for our understanding of global ocean productivity and our ability to predict Earth's future climate. 

Last Modified: 03/29/2026
Modified by: William M Landing